Washing apparatus, and washing method

ABSTRACT

Disclosed herein is a washing apparatus comprising a washing chamber, an opening/closing mechanism, and a nozzle. The chamber has an opening and is designed for washing the objects transported from outside through the opening. The opening/closing mechanism is designed to open and close the opening of the washing chamber. The nozzle is used to wash the opening/closing mechanism. The apparatus further comprises a washing vessel filled with a washing liquid, for washing the objects, and also a nozzle for applying a washing liquid to the objects located in the washing vessel when the objects are partly exposed as the washing liquid is discharged from the washing vessel.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus and a method of washingobjects such as semiconductor wafers.

2. Description of the Related Art

In the manufacture of semiconductor devices, semiconductor wafers arewashed by washing machines, whereby process solutions, impurities, andthe like are removed from the wafers. A washing apparatus of this typewashes wafers with ammonia water, with water, and with fluoric acid,thereby cleaning the surfaces of the wafers.

Some problems are inherent in the washing apparatus. First, the liquidin which a wafer has been washed with remains on the surface of thewafer when the wafer is washed with another solution, making itdifficult to wash the wafer thoroughly. Second, the washing liquids, forexample, ammonium and fluoric acid, react with each other, forming saltwhich eventually remain in the form of particles on the surface of thewafer and inevitably reduces the yield of semiconductor devices. Third,when the wafer comes out of contact with any washing liquid and isexposed to air, it is dried and may have a stain on its surface, whichwill impair the outer appearance of the wafer and degrade the electriccharacteristic thereof.

Generally it is desirable that each washing liquid be maintained pureand clean in order to increase the washing efficiency. To this end, thewashing liquid discharged from the washing tank is passed through afilter, thereby removing impurities or other liquid as much as possible,and the washing liquid thus cleaned is supplied back into the tank andused again, as is disclosed in Published Unexamined Japanese PatentApplication No. 62-94939. The filtration, however, cannot clean thewashing liquid completely.

SUMMARY OF THE INVENTION

An object of this invention is to provide a washing apparatus, whereinneither a residual washing liquid nor particles remain on semiconductorwafers when the wafers are washed, thereby maintaining a high yield ofsemiconductor devices.

Another object of the invention is to provide a washing apparatus and awashing method which can prevent objects from being stained and can keepwashing liquid clean.

In an aspect of the invention, there is provided a washing apparatuscomprising a washing chamber having an opening and designed for washingthe objects transported from outside through the opening,opening/closing means for opening and closing the opening of the washingchamber, and washing means for washing the opening/closing means.

In another aspect of the invention, there is provided a washingapparatus comprising a washing vessel for containing a washing liquidand designed to wash objects with the washing liquid, liquid-dischargingmeans provided in the washing vessel, and liquid-applying means forapplying the washing liquid to the objects located in the washing vesselwhen the objects are partly exposed as the liquid-discharging meansdischarges the liquid from the washing vessel.

In still another aspect of the invention, there is provided a washingmethod comprising the steps of: supplying a washing liquid to a washingvessel, immersing objects in the washing liquid contained in the washingvessel, discharging the liquid at least once from the washing vessel,and applying the washing liquid to the objects when the objects are atleast partly exposed as the liquid is discharged from the washingvessel.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1 is a layout showing a washing system to which this invention isapplied;

FIG. 2 is a perspective view showing a washing tank incorporated in awashing apparatus according to the invention;

FIG. 3 is a plan view of the washing section of the washing apparatus;

FIG. 4 is a cross-sectional view illustrating the boundary between thewashing section and transport section of the apparatus;

FIG. 5 is an enlarged cross-sectional view of the boundary between thewashing section and the transport section;

FIG. 6 is a cross-sectional view showing a washing tank equipped with aliquid-jetting mechanism;

FIG. 7 is a diagram showing the liquid-jetting mechanism located at itsretreated position;

FIGS. 8A, and 8C are diagrams explaining a washing method according tothe invention;

FIG. 9 is a schematic view showing the liquid-circulating system used inthe washing apparatus;

FIG. 10 is a timing chart, explaining how a back pressure is controlledin the system shown in FIG. 9;

FIG. 11 is a perspective view showing a washing tank having a mechanismfor preventing wafers from floating;

FIG. 12 shows the seal mechanism used in the washing tank shown in FIG.11;

FIG. 13 is a diagram illustrating a floating-preventing member ofanother type which can be used in the mechanism of FIG. 11;

FIG. 14 is a diagram showing a mechanism of another type for preventingwafers from floating;

FIG. 15 is a cross-sectional view showing the seal mechanism used in themechanism of FIG. 14;

FIG. 16 is a perspective view illustrating the transport unitincorporated in the washing system of FIG. 1;

FIG. 17 is a plan view of the loading/unloading station used in thetransport unit; and

FIG. 18 is a diagram explaining how the transport unit handlessemiconductor wafers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An washing apparatus according to this invention will now be described,with reference to the accompanying drawings.

FIG. 1 is a layout of a washing system which incorporates the washingapparatus. The system comprises a washing unit 100 and a transport unit200. The unit 100 is designed to wash semiconductor wafers, and the unit200 to transport wafers into and out of the washing unit 100.

The washing unit 100 comprises washing sections 12, 13, and 14 which arearranged in a straight line in the order mentioned. The first washingsection 12 has a loader 15 for loading semiconductor wafers. The loader15 has a stage for holding two carriers D each capable of holding atmost 25 wafers. The carriers D containing wafers, which are to bewashed, are transported to the loader 15 and placed on the stage, atprescribed positions, by means of a robot (not shown).

The third washing unit 14 has an unloader 16 for unloading semiconductorwafers. The unloader 16 has a stage for holding the carriers D. Thecarriers D containing wafers, which have been washed, are transportedfrom the unloader 16 to the transport unit 200 by means of the robot(not shown).

The first washing section 12 further has a transport chamber 31, anunderwater loader 17, and two washing tanks 20 and 21. The chamber 31 islocated in the center part of the section 12 and beside the loader 15.The underwater loader 17 and the washing tanks 20 and 21 are arranged onthe three sides of the transport chamber 31. The chamber 31 contains atransport mechanism 19 having a multi-joint arm 40. The mechanism 19 hasa base connected to the shaft of a stepping motor (not shown). Hence,when the base is driven by the motor, the arm 40 rotates and expands orcontracts to move semiconductor wafers into and out of the underwaterloader 17 and the washing tanks 20 and 21.

The washing tank 20 is filled with water, and the washing tank 21 isfilled with, for example, ammonia water. The tank 20 is used to achieveso-called "quick damp rinse (QDR)," i.e., water-washing for removingammonia from wafers. The underwater loader 17 is located beside thesecond washing section 13 and is filled with water, for temporarilyholding wafers supplied from the tank 20 by the transport mechanism 19.The semiconductor wafers are transported to the second washing section13 through underwater loader 17.

The second washing section 13 comprises a transport chamber 32, anunderwater loader 18, and two washing tanks 23 and 24. The chamber 32 islocated in the center part of the section 13 and beside the underwaterloader 17 of the first washing section 12. The underwater loader 18 andthe washing tanks 23 and 24 are arranged on the three sides of thetransport chamber 32. The chamber 32 contains a transport mechanism 22having a multi-joint arm 40. The mechanism 22 is identical to themechanism 19 in the transport chamber 31 of the first washing section12, and designed to move semiconductor wafers into and out of theunderwater loaders 17 and 18 and the washing tanks 23 and 24.

The washing tank 23 is filled with, for example, fluoric acid, and thewashing tank 24 is filled with water. The tank 24 is used as a so-called"water-washing overflow tank," for removing fluoric acid from the wafersmoved from the washing tank 23. The underwater loader 18 is locatedbeside the third washing section 14 and is filled with water, fortemporarily holding wafers supplied from the tank 24 by the transportmechanism 22. The semiconductor wafers are transported to the thirdwashing section 14 through underwater loader 18.

The third washing section 14 comprises a transport chamber 33 and adrying tank 27. The transport chamber 33 is located in the center partof the section 13 and beside the underwater loader 18 of the secondwashing section 13. It contains a transport mechanism 25 having amulti-joint arm 40. The mechanism 25 is identical to the mechanism 19 inthe transport chamber 31 of the first washing section 12, and designedto move semiconductor wafers into and out of the underwater loader 18and the drying tank 27. The drying tank 27 is used to perform IPA(isoproplyl alcohol) drying on the wafers which the mechanism 25transports from the underwater loader 18 of the second washing section13.

The underwater loaders 17 and 18, the washing tanks 20, 21, 23 and 24,and the drying tank 27 have essentially the same structure shown in FIG.2. As is shown in FIG. 2, they are placed each within a process chamber34. The chamber 34 has a window 35 through which to move wafers in andout, and a shutter 36 for opening and closing the window 35. The loaders17 and 18, the washing tanks 20, 21, 23 and 24, and the drying tank 27have a wafer boat 37 and an elevator mechanism (not shown) each.

The wafer boat 37 is made of quartz and can be moved up and down by theelevator mechanism. It has three parallel rods 37a. The rods 37a havegrooves spaced apart at predetermined intervals and can support at most50 wafers w, such that the wafers w are held vertically, each having itslower edge fitted in the corresponding grooves of the parallel rods 37a.

As is shown in FIG. 3, the multi-joint arm 40 of the transport mechanism19 can rotate, expand and contract in a horizontal plane. The arm 40 hasa fork 41 attached to its free end. The fork 41 has a pair of supportrods 43 which extend parallel to each other and which have groovesspaced apart at predetermined intervals. These rods 43 can supportsemiconductor wafers W, such that the wafers W are held vertically, eachhaving its lower edge fitted in the corresponding grooves of the rods43. The fork 41 can move and be located right above the tanks 17, 20 and21 when the multi-joint arm 40 is rotated and expanded. To transfer thewafers W from the fork 41 onto the wafer boat 37, the support rods 43 ofthe fork 41 are placed among the parallel rods 37a of the wafer boat 37,and the wafer boat 37 is moved up. To transfer the wafers W from thewafer boat 37 to the fork 41, the support rods 43 of the fork 41 areplaced among the parallel rods 37a of the wafer boat 37, and the waferboat 37 is moved down.

It is desirable that the rods 37a and 43 be made of material which issoft and anti-corrosion, such as fluorine-series resins. If the wafers Whave been washed with a strong-acid or strong-alkali liquid, the rods37a and 43 should be made of ethylene fluoride resins, preferablytetrafluoroethylene resin or trifluoroethylene resin. If the wafers Whave been washed with a weak-acid or weak-alkali liquid, the rods 37aand 43 should be made of vinylidene fluoride resin.

The first washing section 12 will now be described in greater detail,with reference to FIG. 4 which is a cross-sectional view showing the thetransport chamber 31 and the process chamber 34 containing the washingtank 20.

The process chamber 34 communicates with the chamber 31 via the window35. The shutter 36, designed to open and close the window 35, is made ofanti-chemical material such as quartz glass, slidably inserted in a case38 attached to the process chamber 34 and located at the window 35, andcan be moved up and down. The case 38 is made of water-proof andanti-corrosion material such as vinyl chloride.

As is evident from FIG. 4, the shutter 36 is coupled to the plunger 52of an air cylinder 51 which is located below the case 38 and used asmeans for driving the shutter 36. The shutter 36 is moved up to closethe window 35, when the plunger 36 is moved outwards. The shutter 36 ismoved down to open the window 35, when the plunger 36 is moved inwards.

In the transport chamber 31, the multi-joint arm 40 of the transportmechanism 19 is driven by the stepping motor 45. As has been described,the fork 41 rotatably attached to the free end of the arm 40 moves thewafers W into and out of the chamber 34.

A plate 53 having a number of holes is arranged above the transportchamber 31. A duct 54 is connected to the plate 53. In the duct 54, anHEPA filter 55 is fixed, and an air-supplying fan 56 is located abovethe HEPA filter 55. The bottom of the transport chamber 31 has two ports57 through which to discharge air and liquid. A plate 58 having holes isfixed in the chamber 31, right above the bottom thereof, for controllingthe flow of air and liquid. A fan (not shown) is provided in either port57, for promoting the discharge of air and liquid.

When the air-supplying fan 56 and the fans (not shown) set in the ports57 are driven, cleaned air is supplied into the transport chamber 31,flows downwards in the form of a layer, passing through the chamber 31,and is discharged from the chamber 31 through the ports 57 made in thebottom of the chamber 31. Hence, the atmosphere in the chamber 31remains clean.

The other transport chambers 32 and 33 of the washing unit 100 havestructures similar to that of the transport chamber 31. In thesechambers 32 and 33, too, clean air flows downwards, maintaining theatmosphere in the chambers 32 and 33 clean.

FIG. 5 is an enlarged cross-sectional view of the boundary between thetransport chamber 31 and the process chamber 34. As is shown in thisfigure, an air-supplying plate 59 having a number of small holes isfitted in the upper end of the case 38, and a duct 60 is connected tothe upper end of the case 38. The duct 60 contains a HEPA filter 61 andan air-supplying fan 62. The filter 61 is mounted on the air-supplyingplate 59. The fan 62 is located above the filter 61. The lower end ofthe case 38 has a port 63 for discharging air and liquid from the case38. A discharge pipe 64 is connected to the port 63 at one end, and to asuction means (not shown) at the other.

When the air-supplying fan 62 and the suction means are driven, cleanair flows into the case 38 through the air-supplying plate 59, downwardsthrough the case 38, and out of the case 39 via the port 63, forming anair curtain in the case 38. Hence, the air leaking from the processchamber 34 and containing chemicals can be expelled outside from, forexample, the clean room in which the washing system (FIG. 1) isinstalled.

As is shown in FIG. 5, an air-liquid separator 65 is mounted on thedischarge pipe 64. The separator 65 separates the washing liquid and thegases generated in the process chamber 34 from the air discharged fromthe case 38. If necessary, an inert gas or N₂ gas can be supplied intothe case 38 through the air-supplying plate 59.

A shower nozzle 66 is located in the upper-end portion of the case 38,more precisely right below the air-supplying plate 59. The nozzle 66 isconnected to a pipe 67. The pipe 67 extends through the wall of the case38 and is connected to a washing liquid tank 68 which is located outsidethe case 38. The tank 68 contains, for example, pure water. The purewater is supplied from the tank 68, and the shower nozzle 66 sprays thepure water onto the shutter 36 slidably inserted in the case 38, therebyremoving chemicals and impurities from the shutter 36.

As is shown in FIG. 5, too, a seal mechanism 69 is fitted in the bottomof the case 38. The mechanism 69 comprises a hollow guide cylinder 70and two seal members 71. The cylinder 70 penetrates the bottom of thecase 38. The seal members 71 are inserted in the guide cylinder 70 andmounted on the plunger 52 of the air cylinder 51. The cylinder 70 ismade of, for example, vinyl chloride, and the seal members 71 are madeof, for example, synthetic rubber.

A sealed space 71 is formed within the guide cylinder 70 by virtue ofthe seal members 71. The cylinder 70 has an air hole 73. An N₂-supplying pipe 74 is connected to the air hole 73 at one end and to anN₂ tank 75 at the other end. N₂ gas is supplied into the sealed space 72from the tank 75 through the pipe 74. Filled up with the N₂ gas, thespace 72 is pressurized. Thus, the washing liquid is prevented fromflowing into the air cylinder 51 along the plunger 52, to corrode theair cylinder 51. The air cylinder 51 can therefore have a prolongedlifetime.

Various operations are performed within the transport chamber 31, aswill be explained as follows.

First, the plunder 52 is driven into the air cylinder 51. The shutter 36is thereby moved downwards, opening the window 35 of the process chamber34. Next, the multi-joint arm 40 of the transport mechanism 19 isdriven, such that the fork 41 attached to the free end of the arm 40transports the semiconductor wafers W into the process chamber 34.Meanwhile, clean air is applied downwards into the case 38 through theair-supplying plate 59, forming an air curtain. The air curtain preventsthe external atmosphere from entering the process chamber 34 (i.e., thewashing chamber). Now that the wafers W have been placed in the chamber34, the arm 40 of the transport mechanism 19 is driven back into thetransport chamber 31, and remains there until the wafers W are washed.As soon as the arm 40 is moved out of the process chamber 34, along withthe fork 41, the air cylinder 51 is driven, thrusting the plunger 52upwards. The shutter 36 is thereby moved up, closing the window 35 ofthe chamber 34. Hence, the atmosphere in the chamber 34 cannot leakoutside at all.

In the meantime, the shower nozzle 66 applies the washing liquid ontothe shutter 36, whereby the chemicals and impurities (e.g., fineparticles of salt formed by the chemical reaction in the chamber 34) areremoved from the shutter 36. The shutter 36 therefore remains clean atall times and causes no possibility of contaminating the atmosphere inthe process chamber 34.

It is desirable that the shutter 36 be washed while the wafers W arebeing washed in the chamber 34 or while the shutter 36 is being moved upor down after the wafers W have been washed. Alternatively, the shuttercan be washed while it stays in the lower part of the case 38. In thelatter case, the nozzle 66 should better be located below the case 38.

The second washing section 13 and the third washing section 14 may havesimilar structure of that of the first washing section 12.

Also, as has been described, the washing tanks 20, 21, 23 and 24 haveessentially the same structure. Hence, only the washing tank 20 will bedescribed in detail, with reference to FIG. 6.

As is shown in FIG. 6, the washing tank 20 comprises an inner tank 20aand an outer tank 20b. In the inner tank 20a, semiconductor wafers W canbe washed with washing liquid L. The outer tank 20b surrounds the upperrim of the inner tank 20, for receiving the liquid L overflowing theinner tank 20a. A liquid-discharging pipe 81 and liquid-supplying pipes82 are connected to the bottom of the inner tank 20a and communicatetherewith. The liquid L is discharged from the inner tank 20a throughthe pipe 81, and fresh liquid L is supplied into the inner tank 20athrough the pipes 82. As a result, the liquid L in the tank 20a can berenewed and remain clean.

A bubbling pipe 83 is connected to the lower-end portion of the innertank 20b. Nitrogen gas is jetted through the pipe 83 into the liquid Lcontained in the inner tank 20a, thereby bubbling or agitating thewashing liquid L. The outer tank 20b has a port 95 through which todischarge the washing liquid L.

A washing pipe 85 is located above the washing tank 20. The pipe 85 hasa plurality of nozzles 84 which are arranged in, for example, two rows.The pipe 85 is attached to a vertical arm 87. The arm 87 is connected tothe shaft 86a of a reversible motor 86, which extends horizontally. Whenthe motor 86 is driven, the arm 87 is rotated in a vertical plane,whereby the nozzles 84 can be placed right above the inner tank 20a andright above the outer tank 20b. To wash the wafers W held in the innertank 20a, the arm 87 is rotated, bringing the nozzles 84 to the washingposition shown in FIG. 6, and the liquid L is applied from the nozzles84 in the form of spray or shower, onto the surfaces of thesemiconductor wafers W. After the wafers W have been washed, the arm 87is rotated, thus moving the nozzles 84 from the opening of the innertank 20a to a position right above the outer tank 20b, as is shown inFIG. 7. Hence, the pipe 85 or the arm 87 does not hinder the transportof the wafers W into or out of the inner tank 20a.

The washing liquid L is supplied to the pipe 85 from a washing liquidtank 88 via a liquid-supplying pipe 89 connecting the pipe 84 to thetank 88. The liquid L supplied to the pipe 84 is jetted out through thenozzles 84.

A bypass pipe 90 is connected to the liquid-supplying pipe 89 at bothends. A four-port, two-position valve 91 is mounted on theliquid-supplying pipe 89 and the bypass pipe 90, for selecting the pipe89 or the bypass pipe 90. A throttle valve 92 is mounted on the bypasspipe 90, for making the liquid L flow through the bypass pipe 90 at arate lower than the rate at which the liquid L flows through theliquid-supplying pipe 89.

Three liquid-level sensors 93 are mounted on the outer side of thewashing liquid tank 88. These sensors 93 are of electrostaticcapacitance type and can detect the level of the liquid L in the tank88, though they do not contact the liquid L at all. Since they do notcontact the liquid L, their lifetime is longer than that of liquid-levelsensors of any other type, and do not breed bacteria in the washingliquid L. Since the sensors 93 are of electrostatic capacitance type,the washing liquid tank 88 can be made of any non-metal material, so asnot to cause the sensors 93 to make errors.

It will now be explained how the wafers W are washed in the washing tank20, with reference to FIGS. 8A to 8C.

First, as is shown in FIG. 8A, the wafers W (e.g., 25 wafers) arevertically held on the parallel rods 37a of the wafer boat 37. The waferboat 37 is lowered into the washing tank 20, thereby immersing thewafers W in the washing liquid L contained in the tank 20. Next, thehydrogen gas is supplied from a nitrogen gas source (not shown) throughthe bubbling pipe 83 into the inner tank 20b, thus bubbling or agitatingthe washing liquid L in the tank 20a. The liquid L is bubbled for apredetermined period of time, washing the wafers W for the first time.

During the first washing of the wafers W, the nozzles 84 are kept abovethe outer tank 20b, and the liquid L dropping from the nozzles 84 arereceived in the outer tank 20b. The liquid L may overflow the inner tank20a as it is bubbled, in which case the overflowing liquid L flows intothe outer tank 20b and will be circulated, along with that liquid Lfallen from the nozzles 84, by means of a liquid-circulating system(later described).

After the first washing of the wafers W, the valve (not shown) providedon the liquid-discharging pipe 81 is opened, thereby discharging theliquid L from the inner tank 20a through the pipe 81, as is shown inFIG. 8B. When the discharging of the liquid L is started, the motor 86is driven, thereby moving the nozzles 84 to a position above the innertank 20a. Then, the nozzles 84, now located above the inner tank 20a,sprays pure water onto the wafers W which are exposing gradually exposedas the washing liquid L is discharged from the tank 20a through the pipe81. The wafers W are thereby washed with the pure water. When thewashing liquid L is completely discharged from the inner tank 20a, thevalve 91 is switched, thereby introducing fresh washing liquid L intothe inner tank 20a through the liquid-supplying pipes 82. Hence, theliquid level in the inner tank 20a rises gradually. When the wafers Ware immersed completely in the fresh liquid L, the nozzles 84 stopspraying the pure water.

When the inner tank 20a is filled up with the fresh washing liquid L,the motor 86 is driven, moving the nozzles 84 to the position above theouter tank 20a, as is shown in FIG. 8C. This done, hydrogen gas issupplied from the nitrogen gas source via the bubbling pipe 83 into theinner tank 20b, thus bubbling or agitating the washing liquid L in thetank 20a. The liquid L is bubbled for a predetermined period of time,washing the wafers W for the second time.

The wafers W are further washed a prescribed number of times, in thesame way as described above.

Since pure water is applied to the wafers W all time the washing liquidL is discharged from the inner tank 20a, they are not exposed to air.Thus, the wafers W are not dried as long as they remain in the washingtank 20, and no stains will be formed on the semiconductor wafers W.

The liquid-circulating system connected to the washing tank 20 will nowbe described, with reference to FIG. 9.

As is shown in FIG. 9, the outer tank 20b has a liquid-discharging port95 in its bottom. The port 95 is connected by a pipe 96 to theliquid-supplying port 97 made in the bottom of the inner tank 20a.Hence, the inner tank 20a, the outer tank 20b, and the pipe 96constitute a closed loop through which the washing liquid L cancirculate. A reciprocating pump 98, a damper 99, and a filter 101 aremounted on the pipe 96. The liquid L overflowing the inner tank 20aflows into the outer tank 20b, and is supplied back into the inner tank20a through the port 95, the pipe 96, and the port 97. The pump 98promotes the circulation of the liquid L. The damper 99 and the filter101 cooperate to dampen the pulsations of the liquid L.

More specifically, the reciprocating pump 98 is a bellows pump of theknown type; it has a primary port 98a and a secondary port 98b. The pump98 comprises a hollow cylinder closed at both ends, a fixed partition108 and two movable partitions 106a and 106b. The partition 108 islocated in the middle portion of the hollow cylinder. The movablepartition 106a is located on one side of the partition 108 and connectedthereto by bellows 105a. The movable partition 106b is located on theother side of the partition 108 and is connected thereto by bellows105b. The partition 106a , the left end of the hollow cylinder, and theinner surface of the cylinder define a pressure chamber 104a. Similarly,the partition 106b, the right end of the hollow cylinder, and the innersurface of the cylinder define a pressure chamber 104b. The bellows105a, the movable partition 106a, and the fixed partition 108 define aliquid chamber 107a. Similarly, the bellows 105b, the movable partition106b, and the partition 108 define a liquid chamber 107b. The pressurechambers 104a and 104b have air-supplying ports 103a and 103b,respectively, which are connected by a three-port, two-position solenoidvalve 102. The solenoid valve 102 is switched, such that compressed airis supplied into the chamber 104a through the port 103a and dischargedfrom the chamber 104b via the port 103b, and vice versa. As a result,the movable partitions 106a and 106b are moved back and forth, while thebellows 105a and 105b are alternately expanded and contracted,respectively. In other words, the liquid chambers 107a and 107b arealternately expanded and contracted, respectively, whereby the liquid Lsupplied via the primary port 98a is pumped out through the secondaryport 98b to the inner tank 20a.

It should be noted that the partitions 108, bellows 105a and 105b, andmovable partitions 106a and 106b of the reciprocating pump 98 are madeof material which are not corroded by the liquid L, such as fluorineresins.

As can be understood from FIG. 9, the damper 99 is mounted on the pipe96 and located downstream of the reciprocating pump 98. The damper 99has a damper chamber 110 and a bellows 111. The damper chamber 110 hasan inlet port 99a and an outlet port 99b. The inlet port 99a isconnected to the output of the reciprocating pump 98. The outlet port99b is connected to the input of the filter 101. The bellows 111 iscontained in the damper chamber 110, partitioning the chamber 110 into aliquid chamber 110a and a back-pressure chamber 110b. It is made ofmaterial which has such a modulus of elasticity that it can move,following the pulsations of the liquid L flowing via the pipe 96. Theback-pressure chamber 110b is connected to a compressed air source 113by means of a changeover solenoid valve 112 and a pipe 114. Thecompressed air is supplied into the chamber 110b from the source 113through a flow-rate adjusting valve 115 mounted on the pipe 114 and thenthrough the solenoid valve 112 connected to the damper 99. Hence, a backpressure is applied onto the bellows 111, thereby dampening the flowpulsations of the liquid L.

The solenoid valve 112 operates in synchronism with the solenoid valve102. In other words, it is driven by the signals supplied from thesolenoid valve 102 and applies a back pressure to the bellows 111 atsuch timing as to cancel out the flow pulsations of the liquid L, whichhave been caused by the reciprocating pump 98. More specifically, thevalve 112 applies a back pressure varying as is indicated by thewaveform W2 shown in FIG. 10, which is substantially identical inamplitude and substantially opposite in phase to the waveform W1 (FIG.10) indicating the flow pulsations of the liquid L. The flow pulsationsof the liquid L supplied from the damper 99 is therefore almost nil ascan be understood from the waveform W3 shown in FIG. 10.

In the liquid-circulating system described above, the washing liquid Loverflowing into the outer tank 20b flows through the pipe 96, cleanedby the filter 101, and supplied back into the inner tank 20a. In theprocess, the damper 99 dampens the flow pulsations (waveform W1) of theliquid L which have been caused by the reciprocating pump 98, byapplying the back pressure (waveform W2) onto the bellows 111. In otherwords, when the flow rate of the liquid L from the pump 98 is high, thebellows 111 expands such that the liquid chamber 110a temporarily storesthe excessive portion of the liquid L; conversely, when the flow rate ofthe liquid L is low, the bellows 111 contracts such that the excessiveportion of the liquid L is expelled from the liquid chamber 110a intothe pipe 96. This is why the damper 99 effectively dampens the flowpulsations of the washing liquid L.

The filter 101 therefore receives virtually no impacts as the washingliquid L passes through it, and has practically no risk of being damagedor worn. Nor is there no chance for dust stuck to the filter 101 tomingle into the liquid L. In addition, since the liquid L flows stablyinto the inner tank 20a, it does not vibrate the wafers W in the tank20a, ensuring uniform washing of the wafers W.

As has been stated earlier, the washing liquid L in the tank 20a isbubbled, thereby promoting the washing of the wafers W. If the liquid Lis bubbled too vigorously, the wafers W may float or move in the liquidL. According to the invention, the washing tank 20 can be equipped witha mechanism designed to prevent the wafers W from floating or moving.Such a mechanism will be described, with reference to FIG. 11 which is aperspective view showing the washing tank 20 and the mechanism 120 forpreventing the wafers W from floating in the washing liquid L.

As is shown in FIG. 11, the mechanism 120 is arranged above the washingtank 20. The mechanism 120 comprises a rotary actuator 121, a movablemember 122, and a floating-preventing member 125. The actuator 121 islocated outside the tank 20, for driving the movable member 122. Themovable member 122 is coupled to the drive shaft 121a of the rotaryactuator 121 and extends downwards from shaft 121a; it rotates in avertical plane when driven by the rotary actuator 121. Thefloating-preventing member 125 is connected to the lower end of themovable member 122a and extends horizontally. As the actuator 121rotates the movable member 122 through 90°, the member 125 is movedbetween a standby position outside the tank 20 and a operating positionwhere it touches the tops of the wafers W vertically held in the waferboat 37, thereby preventing the wafers W from floating in the washingliquid L contained in the inner tank 20a. The wafers W are held in theboat 37, with their orientation flats F located highest. It is while themember 125 remains in the standby position that the wafer boat 37holding the wafers W is brought into the inner tank 20a. A photosensor160, which comprises a light-emitting element 161 and a light-receivingelement 162, is located above the washing tank 20, for detecting thepresence or absence of the wafers W in the tank 20.

As is shown in FIG. 12, the movable member 122 comprises a shaft 123 andan arm 124. The shaft 123 is connected to the drive shaft 121a of therotary actuator 121 located outside the process chamber 34, ispositioned coaxial with the drive shaft 121a, and extends into theprocess chamber 34 through the hole 34b made in the wall 34a of thechamber 34. The arm 124 is fastened at one end to the shaft 123 by meansof a screw 126. In the chamber 34, the shaft 123 is surrounded by theholder 127 which is fastened to the wall 34a by screws. Outside thechamber 34, the shaft 123 is surrounded by a housing 128 made ofaluminum. Two annular seal members 129 are mounted on the shaft 123 andspaced apart from each other for a predetermined distance. The firstseal member 129 is clamped between the wall 34a and the holder 127, andthe second seal member 129 between the wall 34a and the housing 128. Asealed space 130 is formed between these seal members 129. The space 130is connected to a nitrogen gas tank 131. Nitrogen gas is supplied fromthe tank 131 into the sealed space 130. The pressure within the space130 is therefore high enough to prevent liquids from leaking from theprocess chamber 34 into the rotary actuator 121, and dust from enteringthe chamber 34 from the rotary actuator 121.

As is shown in FIG. 12, too, the drive shaft 121a and the housing 128are surrounded by a cylindrical cover 133 which is connected at one endto the wall 34a and at the other end to the actuator 121. A bearing 133ais fitted in the housing 128, and holds the shaft 123 rotatably.

The floating-preventing member 125 is a rod made of anti-corrosionmaterial such as polyether ketone (PEEK). The member 125 is locatedslightly above, for example 1 mm above, the orientation flats F of thewafers W held in the wafer boat 37, as long as the wafers W are immersedin the washing liquid L contained in the inner tank 20a. When the wafers20 move up as the liquid L bubbled by the nitrogen gas jetted from thebubbling pipe 83, their orientation flats F abut on thefloating-preventing member 125, and are thus prevented from floating.

The floating-preventing member 125 can be located in contact with theorientation flats F of the wafers W all the time the wafers W areimmersed in the washing liquid L in the inner tank 20a. To place themember 125 so, it suffices to position the axis of the shaft 123slightly lower along the vertical line passing the center of the wafersW. Hence, the member 125 can touch the orientation flats F of the wafersW before it reaches its lower dead point. It is desirable that apressure sensor (not shown) detect the pressure the wafers W apply tothe member 125, and that the actuator 121 be driven by the signal outputby the pressure sensor and representing the pressure thus detected.

The floating-preventing member 125 is not limited to a rod. It can be aplate-shaped one 135 as is shown in FIG. 13. The floating-preventingmember 135 has a number of small holes 136, grooves 137 cut in the lowersurface and extending lengthwise of the member 135, and fin-shapedstrips 138 partly fitted in the grooves 137. The strips 138 are made offlexible, anti-corrosion material such as fluorine-based rubber, andeffectively serve to prevent the wafers W from floating upwards.

Since the wafers W are prevented from floating by the member 125, theycan be spaced apart at shorter intervals than otherwise. Therefore, morewafers W can be washed at a time than if the member 125 were not beingused. Further, the parallel rods 37a of the wafer boat 37 can bearranged densely by virtue of the member 125, which would help toenhance the efficiency of washing the semiconductor wafers W.

With reference to FIG. 14, another type of a mechanism 140 forpreventing wafers W from floating will be described, wherein a movablemember and a floating-preventing member are linearly moved between astandby position and an operating position.

As is clearly seen from FIG. 14, the system 140 comprises a rod-lesscylinder 141, a guide rail 142, a crank-shaped connector 143, a movablemember 144, a floating-preventing member 145, and a control section 148.The rod-less cylinder 141, used as drive means, is slidably mounted onthe guide rail 142 which is located outside the process chamber 34 andwhich extends vertically. The connector 143 is fastened to the cylinder141 at one end. The movable member 144, which is a vertical rod, issecured to the other end of the connector 144. The floating-preventingmember 145 is attached to the lower end of the movable member 144.Hence, when the cylinder 141 is moved along the guide rail 142, thefloating-preventing member 145 is moved linearly.

A projection 146 protrudes from the rod-less cylinder 141. A pressuresensor 147 is fixed in place, right below the projection 146. The sensor147 is so positioned that the projection 146 abuts on it the moment thecylinder 141 moves, bringing the floating-preventing member 145 intocontact with the wafers W held within the washing tank 20 set in theprocess chamber 34. When the projection 146 abuts on the pressure sensor147, the sensor 147 generates and outputs a control signal to a controlsection 148. In response to the signal, the section 148 stops thecylinder 141 moving on the guide rail 142. The pressure the member 145exerts on the wafers W is thereby controlled.

The movable member 144 extends through a hole made in the top wall 34aof the process chamber 34. Fitted in this hole is a seal mechanism 150.As is shown in FIG. 15, the mechanism 150 comprises a hollow cylinder151 and two annular seal members 152. The cylinder 151 is fitted in thehole of the top wall 34a. Both seal members 152 are fitted in thecylinder 151, positioned coaxial to each other and spaced apart fromeach other. The movable member 144 extends vertically through theannular seal members 152. The member 144, the cylinder 151 and the sealmembers 152 define an annular space 153. The hollow cylinder 151 has ahole 154, through which nitrogen gas is supplied into the annular space153 from the nitrogen gas source 155 (FIG. 14) which supplies nitrogengas into the inner tank 20a for bubbling the washing liquid L containedtherein. The gas pressure in this space 153 is high enough to prevent aprocess liquid from leaking from the process chamber 34 and dust fromentering into the process chamber 34. A nitrogen gas source other thanthe source 155 can be provided for supplying nitrogen gas into theannular space 153.

The transport unit 200 of the washing system shown in FIG. 1 will now bedescribed, with reference to FIGS. 16, 17, and 18. In the descriptionwhich follows, any carrier for transporting wafers to and from atransfer station 174 (later described) will be called "carrier C," andany carrier for transporting wafers from the section 174 to the washingunit 100 and vice versa will be called "carrier D."

As is shown in FIG. 16 which is a perspective view of the transport unit200, the unit 200 comprises a loading/unloading station 171, a firstcarrier-driving mechanism 172, a robot 173, a transfer station 174, awafer-holding section 175, a second carrier-driving mechanism 176, and awafer-transferring section 177. In the loading/unloading station 171,wafer carriers C, each containing semiconductor wafers W, aretransported from a wafer storage (not shown) and back thereto. Thestation 171 comprises a loading section 181, a transfer section 182, andan unloading section 183. At the loading section 181, the carriers C aretransported from the wafer storage to the transfer section 182. At thetransfer section 182, the carriers C are transferred to and from thefirst carrier-driving mechanism 172. At the unloading section 183, thecarriers C are transported from the transfer section 182, back to thewafer storage. An interface robot IF is located below the lineconnecting the loading section 181 and the unloading section 183. Therobot IF has a hand which can move up and down and rotate around avertical axis. The robot IF transfers the carriers C among the stations181, 182, and 183.

The loading/unloading station 171 will be described in greater detail,with reference to FIG. 17 which is a plan view of the section 171.

As is evident from FIG. 17, the loading section 181 has two stages 181aand 181b, and the unloading section 183 has also two stages 183a and183b. These stages 181a, 181b, 183a, and 183b can support two carriers Ceach. Each stage has a notch which makes it easy for the interface robotIF to place the carrier C on the stage.

A projection 196 protrudes from the back of each stage, so that thecarrier C has its handle T automatically directed in the directionsshown in FIG. 16, when it is mounted on the stage.

The transfer section 182 has a stage 197 which can move in thedirections of arrow A. The stage 197 has guides 198 for positioning twocarriers C on the stage 197, with their handles T facing away from eachother so that the carriers C occupies the least area possible. The stage197 also has notches 197a and 197b, which make it easy for the interfacerobot IF to place the carriers C on the stage 197.

As is shown in FIG. 16, the first carrier-driving mechanism 172comprises a base 184, a movable member 185, and a pair of arms 186. Thebase 184 can move up and down, or in the directions of arrow Z. Themember 185 can move in the directions of arrow X. The arms 186 can movein the directions of arrow Y. The arms 186 hold a carrier C and moves itfrom the stage 197 of the transfer section 182, while the stage 197remains at its downstream position.

The robot 173 is located, facing the first carrier-driving mechanism172. The robot transport 173 can slide on a guide 187, in the directionsof arrow X, for transferring the carriers C to and from the firstcarrier-driving mechanism 172, with the assistance of an intermediatestage 178.

Arranged downstream of the first carrier-driving mechanism 172 areconveyers L1 and L2. The conveyer L1 conveys empty carriers C to acarrier-washing apparatus (not shown). The conveyer L2 conveys washedcarriers C from the carrier-washing apparatus. These empty carriers Chave been transported from the transfer station 174, where the wafers Wto be washed were transferred into the carrier D.

The transfer station 174 comprises two transfer stages 174a and 174bwhich can move in the directions of arrow X. A wafer holder 175 havingtwo wafer-holding arms 175a and 175b is arranged above the transferstation 174.

As is shown in FIG. 18, two wafer-pushing mechanisms 193 and 194 arelocated below the transfer stages 174a and 174b, respectively. Themechanism 193 has a wafer-pushing member 193a and a lift 193b, and isdesigned to push upwards the wafers W put on the transfer stage 174a.Similarly, the mechanism 194 has a wafer-pushing member 194a and a lift194b, and is designed to push upwards the wafers W put on the transferstage 174b. The wafers W pushed up are held by the wafer-holding arms175a and 175b of the wafer holder 175.

Preferably, the transfer stage 174a, the wafer-pushing mechanism 193,and the arm 175a are used for only the carriers C and the wafers Wtherein, and the transfer stage 174b, the wafer-pushing mechanism 194,and the arm 175b are used for only the carriers D and the wafers Wtherein. This prevents cross contamination between any carrier C and anycarrier D, and also cross contamination between any wafer W to be washedand any wafer W already washed.

The wafers W are transferred from the carrier C to the carriers D in thefollowing manner. First, the first carrier-driving mechanism 172transfers the carriers C containing wafers W, which are to be washed,from the transfer section 182 onto the robot 173. The robot 173, nowholding the carriers C, moves to the transfer stage 174a and transfersthem onto the stage 174a. Next, the transfer stage 174a is moved to aposition below the arm 175a. At this position, the wafer-pushingmechanism 193 pushes up the wafers W until the arm 175a holds the wafersW. The wafer-pushing member 193a of the mechanism 193 is lowered. Next,the transfer stage 174a, now supporting the empty carriers C, is movedback to the position where it received the carriers C from the robot173. The empty carriers C are moved onto the conveyer L1 and conveyedthereby to the carrier-washing apparatus (not shown), which washes theempty carriers C.

Thereafter, the transfer stage 174b holding empty carriers C is moved tothe position below the arm 175a. At this position, the wafer-pushingmember 183a of the mechanism 193 pushes up the wafers W, whereby thewafers W are transferred from the arm 175a to the transfer stage 174a.The member 183a is lowered, thus placing the wafers W in the emptycarriers D mounted on the transfer stage 174a.

Conversely, to transport wafers W, already washed, to the unloadingsection 183, the carriers D containing the washed wafers W are put onthe transfer stage 174b. Then, the wafer-pushing mechanism 194 pushesthe wafers W upwards, whereby the wafers W are held by the arm 175a.Meanwhile, empty carriers C, already washed, are mounted on the transferstage 174b. This stage 174a is moved to a position below the arm 175a,where the wafer-pushing member 194a of the mechanism 194 pushes thewafers W upwards, whereby the wafers W are transferred from the arm 175binto the washed carriers C placed on the stage 174b.

The second carrier-driving mechanism 176 is located, facing the transferstation 174. The mechanism 176 comprises a base 188, a movable member185, and a pair of arms 190. The base 188 can move up and down, or inthe directions of arrow Z. The member 185 can move above the base 188,in the directions of arrow X. The arms 190 extend parallel to eachother, and can move in the directions of arrow Y. Hence, the mechanism179 can transfer carriers D containing wafers W which are to be washedor already washed, between the transfer station 174 and thewafer-transferring section 177.

It will now be explained how the washing system of FIG. 1 operates towash semiconductor wafers.

At first, carriers C, each containing wafers W to be washed, aretransported to the loading section 181 of the transport unit 200. Thecarriers C are transported from the section 181 to the transfer station174 through the transfer section 182, by means of the firstcarrier-driving mechanism 172 and the robot 173. In the station 174, thewafers W are transferred from the carriers D to the washing unit 100.The second carrier-driving mechanism 176 also transports the carriers C,now empty, to the wafer-transferring section 177.

The robot (not shown) installed in the washing unit 100 conveys thecarriers D, two at a time, from the wafer-transferring section 177 tothe carrier-holding stage of the loader 15 incorporated in the washingunit 100. The wafer-aligning mechanism (not shown) of the loader 15aligns the wafers W in both carriers D placed on the carrier-holdingstage, such that the orientation flats F are positioned highest. Next,the wafers W, for example 50 wafers, are pushed up, out of the carrierD, and transferred onto the fork 41 attached to the free end of the arm40. The arm 40 is moved away from the loader 15 to the transport chamber31 of the first washing section 12, thereby transporting the wafers W tothe transport chamber 31.

Next, the arm 40 is rotated and expanded, positioning the fork 41 infront of the window 35 of of the process chamber 34 and also aligningthe fork 41 with the wafer boat 37 located in the tank 21. This done,the shutter 36 is opened. The arm 40 is moved into the washing tank 21,thus moving the fork 41 into the boat 37. As a result, the wafers W aremounted on the boat 37. The arm 40 is pulled out of the chamber 34through the window 35; the fork 41 is also moved out of the chamber 34.Thereafter, the shutter 36 is closed, and the wafer boat 37 is lowereduntil the wafers W are immersed in the ammonia water contained in thewashing tank 21.

Upon lapse of a prescribed time, the wafers W are pulled up out of theammonia water. Then, the window 35 is opened, and the fork 41 is broughtinto the process chamber 34 and lowered in the tank 21. The wafers W,washed with the ammonia water, are mounted on the fork 41. The fork 41is moved from the chamber 34 through the window 35, thus returning thewafers W into the transport chamber 31.

Then, the arm 40 is rotated and expanded, such that the fork 41 holdingthe wafers W is positioned in front of the window 35 of the processchamber 34 containing the water-washing tank 20. The shutter of 36 isopened, and the fork 41 is inserted into the tank 20 through the window35. The wafers W are transferred from the fork 41 onto the wafer boat37. The boat 37 is lowered until the wafers W are immersed in the watercontained in the tank 20.

Upon lapse of a predetermined time, the boat 37 is pulled out of thewater. The wafers W, water-washed, are transferred from the boat 37 ontothe fork 41. The arm 40 is rotated, thereby moving the fork 41 to theunderwater loader 17.

The transport mechanism 22 of the second washing section 13 is driven,taking the wafers W from the underwater loader 17 into the transportchamber 32 of the second washing section 13. The wafers W aretransferred from the fork 41 onto the wafer boat 37 located above thewashing tank 23. The boat 37 is lowered until the wafers W are immersedinto the fluoric acid solution filled in the tank 23. Upon lapse of apredetermined time, the boat 37 is lifted, thus taking the wafers W outof the fluoric acid solution. The wafers W are then transferred from theboat 37 onto the fork 41. The fork 41 is moved into the transportchamber 32, thus bringing the wafers W, washed with the fluoric acidsolution, into the chamber 32.

Further, the arm 40 is rotated and expanded, positioning the fork 41right above the water-washing tank 24. The wafers W are then transferredfrom the fork 41 onto the wafer boat 37 located in the tank 24. The boat37 is lowered until the wafers W are immersed into the water containedin the tank 24. While the wafers W are being washed with water, thenozzles 84 apply pure water onto the rods 43 of the fork 41, thusremoving the residual fluoric acid from the rods 43. Next, the wafers Ware transferred from the boat 37 onto the fork 41. The fork 41 is movedfrom the tank 24 to the underwater loader 18. The fork 41 is loweredinto the loader 18, whereby the wafer w are placed within the underwaterloader 18.

Thereafter, the transport mechanism 25 of the third washing section 14transports the wafers W from the underwater loader 18 to the drying tank27. In the tank 27, the wafers W are dried, by using isopropyl alcohol.The fork 41 of the mechanism 25 takes the wafers W, thus dried, to thecarriers D mounted on the unloader 16. The wafers W are placed in thecarriers D.

The robot (not shown) transports the carriers D from the unloader 16 tothe wafer-transferring section 177 of the transfer unit 200. The secondcarrier-driving mechanism 176 takes the wafer carriers D from thesection 177 to the transfer station 174. At the station 174, the wafersW are transferred from the carriers D to the carriers C which have beenwashed. The robot 173 and the first carrier-driving mechanism 172cooperate, transporting the carriers C to the transfer section 182. Thecarriers C are transported from the section 182 to the unloading section183, and are hence moved into the wafer storage (not shown).

Thus ends the entire operation of the washing system shown in FIG. 1. Itis desirable that all steps of washing wafers W be performedautomatically in accordance with programs which run on a computersystem.

In the embodiment described above, each wafer boat 37 used is moved upand down. Instead, the wafer boat 37 can be secured to the process tank.Further, the method of opening and closing the shutter 36 is not limitedto the one explained above; the shutter 36 can be moved up or slidsideways, to open the window 35 of the process chamber 34. Moreover, theposition of the nozzles 66 is not limited to the one specified above;the nozzles 66 can be located at any other position provided it canapply pure water onto the shutter 36. Still further, the shutter 36 canbe washed while it is kept immersed in a washing liquid contained in thelower part of the case 38.

According to the present invention, the objects can be washed in eachprocess tank by any method, provided that the objects are immersedduring the washing process.

The washing system described above is designed to wash semiconductorwafers W. Nonetheless, the present invention can be applied toapparatuses and methods of washing LCD substrates, printed circuitboards, and the like.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details, and representative devices, shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

What is claimed is:
 1. A washing apparatus comprising:a washing vesselfor containing a washing liquid and designed to wash objects with thewashing liquid when the objects are immersed in the washing liquid;liquid-discharging means provided in said washing vessel; andliquid-applying means for applying the washing liquid to the objectslocated in said washing vessel when surfaces of the objects are at leastpartially exposed as the liquid-discharging means discharges the liquidfrom said washing vessel to prevent the surfaces of the object fromdrying and staining; wherein said liquid-applying means comprises anozzle for applying the washing liquid to the objects, and saidliquid-applying means has drive means for moving said nozzle between afirst position where said nozzle applies the washing liquid to theobjects when the surfaces of the objects are at least partially exposedas the liquid discharging means discharges the liquid from said washingvessel, and a second position outside said washing vessel and above anouter vessel where a small amount of washing liquid is dropped from saidnozzle to said outer vessel.
 2. The apparatus according to claim 1,wherein said washing vessel has a liquid-supplying port for supplyingthe washing liquid into said washing vessel.
 3. The apparatus accordingto claim 1, further comprising bubbling means for bubbling the washingliquid contained in said washing vessel.
 4. The apparatus according toclaim 1, further comprising floating-preventing means for preventing theobjects from floating up in the washing liquid contained in said washingvessel.
 5. The apparatus according to claim 4, wherein saidfloating-preventing means has a floating-preventing member located rightabove the objects, for preventing the objects from floating upwards, andfloating-preventing drive means for moving the floating-preventingmember between a first position where said member prevents the objectsfrom floating upwards and a second position different from the firstposition.
 6. The apparatus according to claim 5, wherein saidfloating-preventing drive means rotates said floating-preventing member.7. The apparatus according to claim 6, wherein said floating-preventiondrive means moves said floating-preventing member.
 8. The apparatusaccording to claim 1, further comprising:a washing liquid supply portformed at a bottom portion of said washing vessel for supplying thewashing liquid to said washing vessel.
 9. The apparatus according toclaim 8, further comprising:circulating means extending-from said outervessel to said washing liquid supply port for circulating the washingliquid in said outer vessel to said washing liquid supply port.